AN1139 STMicroelectronics, AN1139 Datasheet - Page 32

• Manufacturer Part Number: AN1139
• Description: L6254 - L6268 - L6269 12V DISK DRIVE POWER COMBO IC
• Manufacturer: STMicroelectronics

AN1139 APPLICATION NOTE
loop formed by the error amplifier, power amplifiers and current sense amplifier is set by 4 times the value of
the external current sense resistor (in series with the VCM), and the ratio of the two summing resistors Ri
and Rf. The open loop bandwidth of the Error
amplifier is 10MHz with a gain of 100dB. The open loop bandwidth of the Power Stage is 1MHz for A+ and
1MHz for A- with a gain of +8 for A+ and -8 for A-. Therefore the total gain of the power stage is 16. The open
loop bandwidth of the Sense amplifier is 10MHz and the gain is set to 4.
If the Capacitor Cc2 (Figure #13) is choosen to be 10 times smaller than Cc1, the pole created by Cc2 is further
reason the Cc2 capacitor will not be included in the Transfer Function and the equation can be simplified as
follow.
•
The single Pole response is independent from Rc ( if set
The Bandwidth is set with Cc1:
•
•
5.2.2 PSM Control Loop
The PSM control loop is selected by a ‘1’ in the Reg#0.6 bit. The complete system is described in Figure #14.
The architecture of the current sense, error amplifier and control voltage (from DAC) are the same as for the
linear mode of operation. However the output of the error amplifier (Vx) is input to two comparators which switch
the VCM+ and VCM- outputs ON and OFF.
Figure 14. Voice Coil Psm Control Loop
32/64
DAC
ERROR_IN
13.
14.
15.
Pin 39
Ri
Rf
---- -
V
I
Cc1
VCC/2
o
i
Cc2
Wo
=
-
+
------------------ -
GRiR s
ERROR_OUT
=
Pin 38
AMPLIFIER
R
Io
-----
Vi
Rc
ERROR
f
---------------------------------------------- -
Rf R s
=
----------------------------------------------------------------- -
s
------------------ -
R iGRs
2AGRs
RfCc1 Rs
-----------------------------------------------
+
Rf
Vx
Rm
2 AGR s
Cc1
-
+
-
+
T1
T2
1
+
Rm
A-
SENSE_OUT
+
Pin 37
SENSE AMPLIFIER
1
Cc1 Rc
PIN 26
VCM_A-
VCM Loop Transfer Function
VCM loop Bandwidth
VCM loop low frequency gain
=
SENSE_IN-
G=4
-----------------------
Rs
Rm
Pin 29
Motor
-
+
L m
+
Rm
Lm
).
Rs
Pin 30
VCM_A+
SENSE_IN+
Pin 27
A+